1. Field of the Invention
The present invention relates to a semiconductor laser device for use in, for example, electronic information devices, and more particularly to a semiconductor laser device in which reflectance control films are provided on the end faces of the resonator.
2. Description of the Related Art
In a semiconductor laser, laser oscillation is produced between the front and rear end faces of the resonator so as to emit a laser beam from the front end face, for example. In order to efficiently emit the laser beam, reflectance control films are provided on the front and rear end faces of the resonator to appropriately adjust the reflectance of these end faces.
Specifically, the reflectance control film provided on the front end face is a dielectric film formed so as to reduce the reflectance of the front end face. The reflectance control film on the rear end face, on the other hand, is a dielectric film formed so as to increase the reflectance of the rear end face.
Aluminum oxide films or multilayer films including an aluminum oxide film are used as these dielectric films.
Aluminum oxide has substantially the same coefficient of linear expansion as GaAs, which is a constituent material of semiconductor lasers. Therefore, when an aluminum oxide film is formed on a GaAs surface as a dielectric film, they securely adhere to each other. Furthermore, aluminum oxide has high thermal conductivity. Because of these advantages, an aluminum oxide film is used as a first layer dielectric film adhered to a resonator formed of GaAs.
Especially, a single layer aluminum oxide film is used as a reflectance control film having low reflectance provided on the light emitting front end face of the resonator. Use of a single layer aluminum oxide film is advantageous in that: the reflectance of the film can be set to a desired value by adjusting its thickness; and the manufacturing process can be shortened, as compared to multilayer configurations in which a plurality of layers are laminated to one another.
On the other hand, a reflectance control film having high reflectance is provided on the rear end face of the resonator. A multilayer film formed of aluminum oxide and silicon is used as this reflectance control film having high reflectance. Since aluminum oxide exhibits good adhesion to a GaAs surface, an aluminum oxide film having a thickness corresponding to an optical length of an integer multiple of λ/4 is used as the first layer dielectric film of the multilayer film adhered to the rear end face of the resonator, where λ is the wavelength of the laser beam. It should be noted that layers sequentially formed on an end face of the resonator are hereinafter referred to as a first layer, a second layer, a third layer, and so on, and the last layer, or the outermost layer, in contact with the external medium is hereinafter referred to as the top surface layer.
The second layer dielectric film formed on the first layer dielectric film is selected to be a silicon film having a thickness corresponding to an optical length of λ/4. Further, the third and fourth layer dielectric films formed on this silicon film are selected to be an aluminum oxide film and a silicon film, respectively, having a thickness corresponding to an optical length of λ/4. Further, an aluminum oxide film having a thickness corresponding to an optical length of λ/4 is formed as the fifth layer dielectric film in contact with the external medium (for example, air).
However, when an aluminum oxide film is placed in an elevated temperature and humidity environment for a certain period of time, moisture enters the film, thereby greatly changing the reflectance of the film from its initial value observed immediately after the formation of the film. Likewise, under the same conditions, moisture also enters an aluminum nitride film, thereby greatly changing the reflectance of the film from its initial value observed immediately after the formation of the film.
If the semiconductor laser is used in a hermetically-sealed inert gas or dry air environment, such penetration of moisture into the aluminum oxide film or aluminum nitride film need not be taken into account. Otherwise, however, moisture may enter these films in high ambient humidity.
If the semiconductor laser is caused to oscillate in a high humidity environment, the reflectance of the aluminum oxide film changes as the temperature of the front end region of the semiconductor laser increases, adversely affecting the output characteristics of the semiconductor laser. Furthermore, the change in the reflectance and in the film quality of the aluminum oxide film may lead to COD (Catastrophic Optical Damage) degradation of the emitting end face of the semiconductor laser.
COD degradation refers to a phenomenon in which a film formed on an end face of a semiconductor laser resonator generates heat and thereby heats up as a result of absorbing the laser beam, leading to melting of the film and eventually to a breakdown of the resonator end face.
In one known example of a configuration of a reflectance control film, the end face protective film (or reflectance control film) on the laser light emitting end face side of a semiconductor laser chip includes: a first layer Al2O3 film having a thickness corresponding to an optical length of λ/4; a second layer SiO2 film having a thickness corresponding to an optical length of λ/4; a third layer Al2O3 film having a thickness corresponding to an optical length of λ/4; and a fourth layer SiO2 film having a thickness corresponding to an optical length of λ/4. (See, for example, the upper-right column on page 2 and FIG. 1 of Japanese Patent Laid-Open No. 3-259585 (1991).)
In another known example, the low reflective film on the laser light emitting end face side of a red semiconductor laser chip having an oscillation wavelength λ of 660 nm includes: a first layer Al2O3 film having a refractive index n1 of 1.638 and a thickness corresponding to an optical length of λ/4; second and fourth layer SiO2 films having a refractive index (n2, n4) of 1.489 and a thickness corresponding to an optical length of λ/4; and a third layer Ta2O5 film having a refractive index n3 of 2.063 and a thickness corresponding to an optical length of λ/4. (See, for example, paragraphs [0019] to [0020] and FIG. 1 of Japanese Patent Laid-Open No. 2004-296903.)
In still another known example, the high reflective film on the rear end face of a semiconductor laser chip includes: a first layer Al2O3 film having a thickness corresponding to an optical length of λ/2; a second layer SiO2 film having a thickness corresponding to an optical length of λ/4; a third layer Ta2O5 film having a thickness corresponding to an optical length of λ/4; a fourth layer SiO2 film having a thickness corresponding to an optical length of λ/4; a fifth layer Ta2O5 film having a thickness corresponding to an optical length of λ/4; a sixth layer SiO2 film having a thickness corresponding to an optical length of λ/4; a seventh layer Ta2O5 film having a thickness corresponding to an optical length of λ/4; and an eighth layer SiO2 film having a thickness corresponding to an optical length of λ/2. (See, for example, paragraphs [0040] to [0047] and FIG. 3 of Japanese Patent Laid-Open No. 2004-327581.)
In yet another known example, the multilayer film formed on the front end face of the resonator of a semiconductor laser having an oscillation wavelength of 800 nm includes: a first layer Al2O3 dielectric film having a thickness of d1 and a refractive index n1; a second layer TiO2 dielectric film having a thickness of d2 and a refractive index of n2; and a third layer SiO2 dielectric film having a thickness of d3 and a refractive index of n3. With this arrangement, in order to set the reflectance of the front end face to 13%, the thicknesses and the refractive indices of these dielectric films are set such that n1*d1=0.095, n2*d2=0.20*λ, and n3*d3=0.235. (See, for example, paragraphs [0023] to [0024] and FIG. 2 of Japanese Patent Laid-Open No. 2001-119096.)
Still another known example relates to an optical transmission device including a nonhermetic optical module in which silicon oxide films are formed on device end faces as oxidation inhibiting films. Since such a device cannot achieve (without special arrangement) sufficient resistance to device degradation under elevated temperature and humidity conditions, the following manufacturing method is used assuming that the layer in contact with the external medium is formed of a silicon nitride film. First, a device having a resonator length of 600 μm is produced through a cleaving process. Then, a reflective film having a reflectance of 95% is formed on the rear end face. This film includes three laminated bodies, each made up of a silicon oxide film having a thickness of λ/(4nSiO2) and an amorphous silicon film having a thickness of λ/(4na-si), where λ is the oscillation wavelength. After that, a silicon nitride film is formed to a thickness of λ/(2nSiNx). As for the front end face, a laminated body formed of a silicon oxide film and an amorphous silicon film such as those described above in connection with the rear end face is formed on the front end face. (This laminated body has a reflectance of 70%.) After that, a silicon nitride film is formed to a thickness of λ/(2nSiNx), as in the case of the rear end face. (See, for example, paragraphs [0007], [0008], and [0012], paragraphs [0028] to [0030], and FIG. 1 of Japanese Patent Laid-Open No. 2000-68586.)
When the reflectance control film on an end face of a resonator is a single layer film of aluminum oxide or aluminum nitride, or when it is a multilayer film including an aluminum oxide film or aluminum nitride film as its top surface layer film, moisture tends to enter the top surface layer film and thereby change its reflectance, as described above, increasing the possibility of COD degradation.
On the other hand, when the reflectance control film is a multilayer film and its top surface layer film is a film having higher moisture resistance than aluminum oxide and aluminum nitride films, for example, a silicon oxide film, COD degradation, due to the change in the reflectance of the top surface layer film caused as a result of moisture entering the top surface layer film, does not necessarily occur. However, since the top surface layer film also constitutes the reflectance control film, it must have a certain thickness, for example, a thickness corresponding to an optical length of λ/4 or λ/2. In this case, if the top surface layer film has lower thermal conductivity than the aluminum oxide or aluminum nitride film under it, the top surface layer film does not provide as much thermal diffusion as the underlayer film, resulting in an increased possibility of COD degradation. Further, it is difficult to select materials for a multilayer film which has a desired reflectance and whose top surface layer film has high moisture resistance, resulting in a greatly reduced degree of freedom in the design of the reflectance control film.